In recent years, a clean fuel-cell powered vehicle, which does not generate any material harmful to the environment, has been under development. The fuel-cell must be cooled by a heat exchanger. During power generation by the fuel-cell, a high voltage is impressed on a cooling medium (such as the cooling water), and therefore the heat exchanger must be electrically insulated.
As a method designed to meet this requirement, the heat exchanger is configured of an insulating material (thermoplastic polymer) (Japanese Unexamined Patent Publication No. 63-311084) or the inner surface of a metal heat exchanger is covered with an insulating material (coated with resin) (Japanese Unexamined Patent Publication No. 2003-181628).
The heat exchanger configured of the insulating material described above, however, has a much lower heat conductivity than the configuration of an aluminum material (about one thousandth of the heat conductivity of aluminum), and therefore the heat transfer performance is deteriorated. The heat conductivity can be increased by mixing a metal material at the sacrifice of a reduced electrical resistance. Thus, as the insulation characteristic cannot be maintained, the heat conductivity cannot be easily improved.
Also, in the heat exchanger of a metal covered with an insulating material as described above, the inner surface of a complicated shape is covered with the insulating material. Depending on the shape, therefore, some parts may not be covered and it is very difficult to apply the insulating material in a state free of pinholes (defects).